FRACTURE SYSTEMS AND DEVELOPMENT IN AN ACTIVE FAULT ZONE

The Little Grand Wash (LGW) Fault, located in Emery County, Utah, is an east-west trending south-dipping anastomosing normal fault system. The area is known for its association with Crystal Geyser, a cold-water CO2 charged geyser emanating from an abandoned borehole drilled within the fault damage zone in 1935. The LGW fault zone provides a natural laboratory to study rock fractures in various lithologies to better understand the controls on rock failure. The fault system contains multiple splays and interaction zones and records a complex history of faulting, fracture creation and reactivation, and multiple fluid flow events. In this project, outcrop exposures are being tied to core recovered in 2019 from the fault zone. The core samples were collected across the Jurassic Brushy Basin Member (JMb) of the Morrison Formation in the hanging wall and the Jurassic Summerville Formation (JS) in the footwall. These core samples and outcrops allow us to compare deformation within the sandstones of the JMb and the siltstones of the JS. Here we combine core observations, including structural descriptions, petrography, and SEM data to understand the mechanical and fracture distribution data that can be tied to the outcrop expression, away from surface weathering processes. These data are supported by computer tomography, where multiple generations of deformation events are identified. We use this data to interpret the controls on fracture development and reactivation including uplift history, early soft-sediment deformation, fluid-rock interactions, and mechanical rock strength changes associated with structural diagenesis and overpressure in the active CO2 fluid system.